1. Field of the Invention
The present invention relates to a filter circuit and a communication device.
2. Description of the Related Art
Recently, along with miniaturization of a complementary metal oxide semiconductor (CMOS) process, the power supply voltage of a radio frequency (RF) circuit has been reduced. In a situation where the CMOS process is miniaturized, if the RF circuit is implemented using a conventional circuit method, the voltage margin becomes insufficient and the dynamic range of the signal amplitude is narrowed. Meanwhile, along with the miniaturization of the CMOS process, the cut-off frequency of a transistor increases. This is advantageous for operations in which high-speed switching operations are performed accurately in terms of time. Further, as the precision of lithography increases, the capacitance ratio of capacitors becomes accurate.
A digital RF technology is a new technology that implements the concept of discrete-time signal processing to the RF circuit, in order to obtain the above-described advantages while avoiding the problems caused by the miniaturization of the CMOS process. A charge domain filter is known as a main circuit in the digital RF technology field. The charge domain filter is a filter circuit formed by a transconductance amplifier, switches and capacitors. The charge domain filter is a circuit which performs sampling of analog signals by accumulating and discharging electric charges in synchronization with a clock, and which performs filtering and decimation by discrete-time signal processing (refer to Japanese Patent Application Publication No. JP-A-2008-18220; Japanese Patent Application Publication No. JP-A-2009-27389; R. Bagheri, “An 800 MHz to 5 GHz Software-Defined Radio Receiver in 90 nm CMOS,” ISSCC Deg. Tech. Papers, pp. 1932-1941, February 2006; Hirata, Takahashi, Kato, Kikui and Takebe, “New high frequency, low-power FIR SC circuit (parallel cyclic type circuit)” Shingakuron (A), vol. 75-A, No. 1, PP. 27-38, January, 1992; and K. Muhammad and R. B. Staszewski, “Direct RF sampling mixer with recursive filtering in charge domain,” in Proceedings of the International Symposium on Circuits and Systems (ISCAS '04), vol. 1, pp. I-577-I-580, Vancouver, BC, Canada, May 2004, sec. ASP-L29.5).